THE IMPORTANCE of site investigation was highlighted on a recent underpinning contract in Worcester Park, south London, where pile design had to be changed after work had started.
Consultant Angell Thompson specified a contractor-designed piled raft underpinning scheme for a garage that had subsided. But its recommendation for a full site investigation was resisted on financial grounds, as generally firm to stiff clays were expected, requiring heave protection.
Minipiling and underpinning contractor Purkelly Bros designed an underpinning raft slab using auger bored piles sleeved to provide heave protection. This, it said, would have been appropriate for open bored piles free of groundwater.
However, boring for the first pile revealed a different picture. Although clay was encountered, cohesion readings were not as high as those assumed in the pile design. Groundwater was also encountered in the bore, the lower part of which collapsed.
Purkelly therefore proposed an alternative piling method using 150mm diameter steel cased driven piles installed in a 3m prebored sleeved hole. This was accepted by Angell Thompson.
Eight piles were installed to 9m using a mini drop hammer rig. Forming the reinforced concrete raft involved breaking out the garage slab and building a 250mm thick slab which cannot be seen above ground level.